There is probably a theme of duality, as suggested by 'dark and light' in the title. It could mean the struggle between good and evil, or different aspects of martial power.
Power is a main theme. The protagonist is constantly striving to gain more power in the martial world of light and dark. Another theme is revenge. He may have some scores to settle which drives his actions.
Power and self - improvement are main themes. The protagonist is constantly striving to become stronger in the martial arts world.
I think the theme of self - discovery is also there. As the character progresses in the martial arts journey, they might discover their true potential, their purpose in this world of martial emperors and gods. Also, friendship and rivalry play a role. There could be comrades who help the main character along the way and rivals who push them to become better.
I'm not sure specifically about 'The Dark is Light Novel' without reading it. But generally in light novels, common themes could be friendship, adventure, and self - discovery. It might also have themes related to the contrast between light and dark, as the title implies.
One similar light novel could be 'Martial World'. It also has elements of martial arts and a protagonist's journey to power in a complex world.
One main theme could be the struggle of the maidens. They might be facing challenges in a dark or difficult world. Another theme could be the power they possess or the lack thereof and how they deal with it.
The main plot of 'Dark Star Emperor Light Novel' often revolves around a character's journey in a dark star - related world. There are likely elements of power - seeking, as the title implies a connection to an emperor. Usually, in such light novels, the protagonist has to face various challenges and enemies to rise to power and dominate the dark star realm.
One of the main themes is probably power. The sword of the emperor is likely a symbol of great power, and the story may revolve around characters seeking or trying to control this power. Another theme could be loyalty, as characters might be loyal to the emperor or the idea of the imperial power associated with the sword.
Another theme might be self - discovery. As the characters progress through their martial arts journey, they also learn more about themselves, their limits, and their true desires.
One of the main themes is the struggle with inner darkness. Dazai's character often battles his own demons. Another theme could be loyalty and betrayal, as seen in the relationships between the characters in the story.